Payment not showing correct. Shows as incomplete when payment can be found

Hello, in the past 6 months I have noticed that there have been times where a customer shows incomplete when in fact they paid. I found 2 such incidents for today’s training event, and 3 last Saturday. This has caused a good deal of issues with the customers who show up for class and dont have a completion certificate because it shows they didn’t pay, when in fact they did. I cannot locate any reason why this is happening. It is causing more work to manually check incomplete registrations (non payment) by logging into authorize.net or paypal. Please let me know how to fix this.